Basic support for building the MPFR
Add more mentions to the MPFR on the build system and documentation
Merge remote-tracking branch 'origin/master' into bigfloat
Add mpc to the build system and basic documentation
Add MPC and MPFR to Base

Both MPC and MPFR are building correctly and running all tests (the
same I had when they were a different package).
Currently, the biggest issue is that `make check` for MPFR and MPC
run all their tests, considerably lengthening the build process.
Change BigInt to use a precise mpz_struct

As discussed in #2814.
Install libmpfr-dev and libmpc-dev packages explicitly
Update MPFRFloat to use a precise sized mpfr_struct

As discussed in #2814. The function `sum` is disabled currently
(so the default one is used as a fallback), as it needs direct
pointer access to each of the members of the array.
Change default precision of MPFRFloat to 256 bits

As discussed in #2814